Rust 2018?

[goddessfreya]

This doesn't seam to come with a recent version of rust.

See https://circleci.com/gh/ZeGentzy/glutin/29#tests/containers/0:

warning: unused manifest key: package.edition

[mb64]

For the short term, #223 introduces a new Docker image with the updates. However, for the long term, there should probably be some automated way of updating it. I will bring it up in #221, which seems to have discussion of CI.